This invention relates to a sensor arrangement for sensing a torque or flexural force in a shaft and to a method and apparatus for inducing a magnetic field in a shaft for sensing by a sensor arrangement. The invention also relates to shafts incorporating transducer elements and to a method of producing a magnetically calibrated shaft for use in a torque sensor.
Magnetic torque sensors are known in the art. They are based on a phenomenon commonly referred to as magnetoelasticity. Known sensors use a magnetized ring secured to a shaft so that torque in the shaft is transmitted to the ring which acts as a transducer element. The sensor also includes one or more magnetic field sensitive elements external to the ring to sense a disturbance of the magnetic field due to the torque resulting from rotation or a torque applied to a non-rotatable shaft. The rotation of any shaft from one end to drive a load at the other end generates torque in the shaft due to differential angular displacement (which may be very small) between the point at which drive is applied and the point at which the load is driven. The same applies to a shaft having one fixed end and another end subject to torque. More particularly to measure this torque the magnetized ring of a magnetic torque sensor has established within it a magnetic field that is essentially confined within the ring. The torque distortion causes magnetic flux to emanate from the ring in proportion to the torque so that the flux can be sensed by one or more external sensing elements.
Various forms of magnetic field sensitive elements have been proposed. One proposal is to sense the small changes in magnetic field due to applied torque, by use of saturable coils (inductors) driven into saturation so that the point of saturation of a coil, with respect to a drive current, depends on the torque-induced external magnetic field associated with the magnetised ring. The points of saturation for the opposite drive polarities in the coil become unbalanced due to the presence of an external field and it is this unbalance that is sensed. Circuitry responsive to the point of saturation is used to develop an output signal representing torque.
Other magnetic field sensing elements have been proposed as is mentioned below.
Thus in general the prior proposed torque sensor comprises two parts: one is the transducer element mounted to the shaft; the other is the sensor element(s) and associated circuitry for driving the elements where necessary and for processing the torque-dependent signals to derive an output representing the measured torque. This other part of the sensor is mountable in a static fashion with the sensor elements closely adjacent to but not contacting the transducer element.
By way of example, a magnetometer using a saturating inductor and the associated circuitry is described in U.S. Pat. No. 5,124,648 (Webb and Brokaw). More detailed information on the mounting of torque sensor coils (inductors) with respect to rotary shafts is to be found in U.S. Pat. No. 5,520,054 (Garshelis). Reference may also be made to a technical paper published by the Society of Automotive Engineers (SAE), xe2x80x9cDevelopment of a Non-Contact Torque Transducer for Electric Power Steering Systemsxe2x80x9d, SAE Technical Paper Series, No. 920707, I. J. Garshelis, K. Whitney and L. May, reprinted from: Sensors and Actuators, 1992 (SP-903), International Congress and Exposition, Detroit, Mich., Feb. 24-28, 1992, pp. 173-182. Further discussion of torque transducers is found in xe2x80x9cA Single Transducer for Non-Contact Measurement of the Power, Torque and Speed of a Rotating Shaftxe2x80x9d, I. J. Garshelis, C. R. Conto and W. S. Fiegel, SAE Technical Paper Series, No. 950536, reprinted from: Sensors and Actuators (SP-1066), International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995, pp. 57-65, particularly pp. 58-59.
An example of a commercially-offered magnetoelastic torque transducer is that offered under the trade mark TorqStar by the Lebow Products division of Eaton Corporation, 1728 Maplelawn Road, Troy, Mich., U.S.A.
Torque sensing arrangements based on magnetoelasticity are described in related U.S. Pat. Nos. 5,351,555 and 5,465,627 Garshelis, assigned to Magnetoelastic Devices, Inc.) and in U.S. Pat. No. 5,520,059 (Garshelis, assigned to Magnetoelastic Devices, Inc.).
U.S. Pat. Nos. 5,351,555 and 5,465,627 are primarily concerned with torque sensing arrangements described in these patents utilize a magnetised ring or torus secured to the shaft, the torque in which is to be measured, and a non-contacting sensor responsive to changes in the magnetic field external to the ring that arise from stresses in the shaft when the shaft is put under torque. The non-contacting torque sensor may use coil assemblies or other devices sensitive to magnetic fields, e.g. Hall effect devices, magnetoresistive devices and so on.
In these patents there is a single circumferential field generated in the ring which in the no-torque state is entirely contained within the ring. The conditions for the establishment of such a field are discussed in these patents. Emphasis is laid on two practical requirements:
1) the ring is endowed with an effective uniaxial magnetic anisotropy having the circumferential direction as the easy axis
2) the ring is subject to hoop stress, that is a stress tending to expand the ring.
There is also a suggestion in these two patents that the separate ring can be implemented in the form of a ring of suitable material cast on to the shaft by spraying or explosively welded on to the shaft, or a ring of surface modified material formed in the shaft as by ion implantation. None of the suggestions is taught in detail and to the present Applicant""s knowledge they have not been put to commercial use.
U.S. Pat. Nos. 5,351,555 and 5,465,627 also give attention to the need to confine the circumferential field within the ring. If the shaft to which the ring is fitted is of low permeability (i.e. paramagnetic), the magnetised transducer ring can be mounted directly to the shaft. However, if the shaft is of high permeability, e.g. ferromagnetic, it is proposed to mount a low permeability spacer between the ring and the shaft. The consideration underlying these proposals appears to be that if the circumferentially magnetised ring were to be mounted directly to a mass of high permeability material, the flux that emanates from the ring under torque would be shunted through the shaft and no useful flux would be available emanating from the ring.
In practice the ring is made with a thin wall thickness, typically 1 mm. for a 20 mm. diameter ring. In order to be pressed onto the shaft and to deal with high torque loads on the shaft, making the ring as thin as possible is advantageous but this has to be balanced against the magnetic field output becoming weaker as the ring becomes thinner. Anothr cost factor to be considered is the need to produce the ring by maching and any special measures taken on the shaft and/or ring to secure the ring against slippage under torque loads add to cost.
U.S. Pat. No. 5,520,059 adds to the disclosure of the two patents discussed above, the concept of the ring transducer having two axially-adjacent zones each having a circumferential magnetisation but the respective magnetisations being of opposite polarity, that is in opposite circumferential directions. More than two zones may be employed of alternating polarity of magnetisation.
There is also a suggestion in U.S. Pat. No. 5,520,059 that a tubular shaft, shown as thin-walled relative to the tube diameter, have a section of the tube itself magnetised to provide a transducer portion of the tube. Specifically the magnetised section has two adjacent zones of opposite polarity magnetisation. However, special measures are taken in this case by inserting a plug into the section of tubular shaft that is to provide the transducer in order to induce hoop stress into that section of the shaft. Furthermore, the magnetised zones should lie axially inward of the ends of the plug. The practical implementation of this proposal requires additional components, i.e. the plugs, and would be difficult to implement without damage and with accuracy of placement, which together with assembly time and entails additional costs.
It is also considered that to define the position of the magnetised region of the tube will probably require additional treatment of the tube adjacent the magnetised region, such as a heat treatment to render the adjacent regions less permeable.
In practice, the torque sensor arrangements of the transducer ring type which have been offered in the market place are of the kind which have a separate transducer ring requiring affixing to the shaft whose torque is to be measured.
The employment of a magnetic transducer element of the torque sensor arrangement in the form of a separate ring that is secured to the shaft such that the torque stresses in the shaft are communicated to the ring entails the following disadvantages:
1. Slippage of the magnetised ring under high torque conditions, impairing reliability;
2. The cost of the magnetised ring;
3. The high tooling cost and manufacturing cost associated with the ring.
4. The high cost associated with using a non-magnetisable material for the shaft. Most shafts, for most torque-transmitting applications, are made of steel which is magnetisable.
5. The high tooling and manufacturing costs associated with the shaft. The shaft needs to have splines, or to be tapered, or have some special tooling to prepare it for receiving the ring.
6. Limitations in the shape and form of the shaft.
The present invention is based on a new concept, namely to use the shaft itself as the magnetoelastic transducer element of a torque sensing arrangement. To this end at least a portion of the shaft itself is magnetised. The invention permits lower manufacturing costs and ease of use as compared with the prior ring transducer technology. The invention also introduces another new concept which is to measure the flexural force in an elongate member subject to a bending moment. For convenience all such elongate members, subject to torque and/or flexure, whether intended for rotation or not, will be referred to as xe2x80x9cshaftsxe2x80x9d.
It is a feature of the concept of the present invention that, contrary to prior thinking, the shaft is of a magnetisable material and is of a solid or substantially solid cross-section at the portion thereof which is to be magnetised. A substantially solid cross-section allows for example an axially-extending bore within the shaft but the bore cross-section being significantly smaller than that of the shaft so that there is a substantial greater thickness of magnetisable material in the cross-section of the magnetised portion than is occupied by the bore. This is acceptable because little of the magnetisation of a circumferentially magnetised shaft penetrates to the central region of the shaft. Typically, with a circular shaft and bore, the bore diameter will not exceed one-third of the shaft diameter. A similar ratio will apply to pertinent dimensions of shafts of non-circular cross-section.
Within the purview of this invention a xe2x80x9csolidxe2x80x9d shaft is to be understood as including a shaft that is solid to an extent substantial enough to satisfy the purposes of the present invention.
In the practice of the invention described below, it will be shown how the transducer portion of the shaft does not require any special treatment, e.g. the inducing of hoop stress or special surface treatment. The magnetised shaft portion is unitary with or integral with the shaft to each side of the magnetised portion in that it is of the same material as the adjoining portions of the shaft section of the shaft relevant to torque measurement, i.e. the shaft is homogeneous as regards its material. This is not to imply a specially high degree of uniformity in the material. It has, for example, been found that normal changes in alloy concentration as they occur in standard steel materials are fully acceptable. In practical engineering applications in many cases a torque transmission shaft is a single unitary object but it is possible to contemplate an overall transmission shaft that is composite in having two or more parts that may be of different characteristics as regards their material but, of course, including a magnetisable part treated in accordance with the present invention. It is a feature of embodiments of the invention to be described below that a shaft of magnetisable material can be circumferentially magnetised at a portion thereof, which may include two or more zones of opposite circumferential polarity in next adjacent zones.
Aspects and features of this invention for which protection is presently sought are set forth in the Claims following this description.